Module # 5: Chapters 22-25
What happens to the brightness of light from each lamp in a series circuit when more lamps are added to the circuit?
Adding more lamps in series produces a greater circuit resistance. This decreases the current in the circuit. All voltages have to add up to the same total voltage, so the voltage drop across each lamp also decreases. Since power is the product of voltage and current, both these changes act to dim the lamps.
A friend says that the reason one's hair stands out while touching a charged Van de Graaff generator is simply that the hair strands become charged and are light enough so that the repulsion between strands is visible. Do you agree or disagree?
Agree with your friend. The hair acts like leaves in an electroscope. If your were as light, they'd stand out too.
When is energy input needed for energy output by electromagnetic induction?
Always
What are the principal differences between an MHD generator and a conventional generator?
An MHD generator has no moving parts.
What is induced by the rapid alternation of a magnetic field?
An alternating field is induced
Series Circuit
An electric circuit in which electrical devices are connected along a single loop of wire such that the same current is in each device.
Parallel Circuit
An electric circuit in which electrical devices are connected in such a way that the same voltage acts across each one, and any single one completes the circuit independently of all the others.
Capacitor
An electrical device—in its simplest form, a pair of parallel conducting plates separated by a small distance—that stores electric charge and energy.
Generator
An electromagnetic induction device that produces electric current by rotating a coil within a stationary magnetic field. A generator converts mechanical energy to electric energy.
Name one particle that has exactly one quantum unit of charge.
An electron (or proton) has exactly one quantum unit of charge.
Can an electron at rest in a stationary magnetic field be set into motion by the magnetic field? What about the electron placed at rest in an electric field?
An electron must be moving across magnetic field lines in order to feel a magnetic force to set it in motion. In an electric field, however, an electron can be accelerated whether or not it is already moving.
Conductor
Any material that has free charged particles that easily flow through it when an electrical force acts on them.
If you place a chunk of iron near the north pole of a magnet, attraction will occur. Why will attraction also occur if you place the same iron near the south pole of the magnet?
Attraction will occur because the magnet induced opposite polarity in a nearby piece of Iron North will induce south and vice versa. this is similar to charge induction, where a balloon will stick to a wall whether positively charged or negatively charged.
A toaster has a heating element of 15 Ω and is connected to a 120-V outlet. Show that the current drawn by the toaster is 8 A. Ohm's Law Formula: I=V/R
Attraction will occur because the magnet induces dirty in a nearby piece alliance. I was willing due south, and south will induce north . This is similar to charge induction, where balloon will stick to the wall whether the balloon is negative or positive.
If a neutral atom has 22 protons in its nucleus, the number of surrounding electrons is a.less than 22. b.22. c.more than 22. d.sometimes any of these in a neutral atom.
B. 22 Comment: Any atom with 22 protons in its nucleus with more or less than 22 electrons is not neutral.
Magnet A has twice the magnetic field strength of magnet B (at equal distance) and, at a certain distance, it pulls on magnet B with a force of 50 N. With how much force, then, does magnet B pull on magnet A?
Back to Newton's 3rd Law! Magnet A and B are pulling equally on each other. If A pulls on B with 50 N, then B pulls on A with 50 N.
Name at least six creatures that are known to harbor tiny magnets within their bodies.
Bacteria, pigeons, bees, butterflies, sea turtles, and fish.
How does the frequency of induced voltage relate to how frequently a magnet is plunged in and out of a coil of wire?
Both frequencies are the same
How is Coulomb's law similar to Newton's law of gravitation? How is it different?
Both laws are inverse square laws. The only difference is that gravitational is only attractive, whereas electrical forces repel.
What is the power in the primary coil?
By the law of conservation of energy, the power in the primary coil is the same, 800 W
How is the direction of an electric field indicated with electric field lines?
Bye convention, the direction goes from positive to negative, as the arrows indicate.
What kind of charging occurs during thunderstorms?
Charging by induction occurs during thunderstorms.
In a circuit of two lamps in series, if the current through one lamp is 1 A, what is the current through the other lamp? Defend your answer.
6 creatures are bacteria, pigeons, bees, butterflies, sea turtles, and fish,
What is the ac current drawn by the primary coil?
8 A, twice as much (100 V × ? A = 800 W).
Assuming the answer to the preceding question is 200 V and the secondary is connected to a flood lamp with a resistance of 50 Ω, what will be the ac current in the secondary circuit?
: From Ohm's law, 200 V/50 Ω = 4 A.
Does a battery produce dc or ac? Does the generator at a power station produce dc or ac?
A battery produces DC. A generator normally produces AC.
What is a magnetic domain?
A cluster of aligned atoms
Will a cyclist coast farther if the lamp connected to the generator on his bicycle is turned off? Explain.
A cyclist will coast farther if the lamp is disconnected from the generator. The energy that goes into lighting the lamp is taking the bike's KE, so the bike slows down. The work saved by not using the lamp will be the extra " force * distance" that lets the bike coast further
Transformer
A device for transferring electric power from one coil of wire to another, by means of electromagnetic induction, for the purpose of transforming one value of voltage into another.
What property of a diode enables it to convert ac to pulsed dc?
A diode passes current in one direction only.
Electricity
A general term for electrical phenomena, much like gravity has to do with gravitational phenomena, or sociology with social phenomena.
Electromagnet
A magnet whose field is produced by an electric current. It is usually in the form of a wire coil with a piece of iron inside the coil.
Maxwell's counterpart to Faraday's law
A magnetic field is created in any region of space in which an electric field is changing with time. The magnitude of the induced magnetic field is proportional to the rate at which the electric field changes. The direction of the induced magnetic field is at right angles to the changing electric field.
Why do clothes often cling together after tumbling in a clothes dryer?
Clothes become charged when electrons from a garment of one material are rubbed into another material. If the materials were good conductors, the materials were good conductors, the discharge between materials would soon occur. But the clothes are nonconducting and the charge remains long enough for oppositely charged garments to be electrically attacked and stick to one another.
Magnetic domains
Clustered regions of aligned magnetic atoms. When these regions themselves are aligned with one another, the substance containing them is a magnet.
At the security area, people walk through a large coil of wire and through a weak ac magnetic field. What is the result of a small piece of metal on a person that slightly alters the magnetic field in the coil?
Eddy currents induced in the metal change the magnetic field which in turn changes the AC current in the coils and sets off the alarm.
Does electric charge flow across a circuit or through a circuit? Does voltage flow across a circuit or is it impressed across a circuit?
Electric charge flows through a circuit. Voltage does not flow at all but is impressed across the circuit.
Conser
Electric charge is neither created nor destroyed. The total charge before an interaction equals the total charge after.
A droplet of ink in an industrial ink-jet printer carries a charge of 1.6 × 10-10 C and is deflected onto paper by a force of 3.2 × 10-4 N. Show that the strength of the electric field to produce this force is 2 million N/C.
Electric field is force divided by charge: E= F/q 3.2*10^-4N/1.6 * 10^-10 C= 2*10^6 N/C
Electric field
Electrical force per unit of charge. The field can be considered to be an "aura" surrounding charged objects and is a storehouse of electric energy. About a charged point, the field decreases with distance according to the inverse-square law, like a gravitational field. Between oppositely charged parallel plates, the electric field is uniform: Electric field = F/q
Direct Current (DC)
Electrically charged particles flowing in one direction only.
Alternating Current
Electrically charged particles that repeatedly reverse direction, vibrating about relatively fixed positions. In the United States, the vibrational rate is commonly 60 Hz.
Why does the word change occur so frequently in this chapter?
Electromagnetic induction requires change- of the intensity of a magnetic field, or of motion in a magnetic field.
Why are electrons, rather than protons, the principal charge carriers in metal wires?
Electrons and metals are for you to wander, whereas protons are imbedded in atomic nuclei, not free to roam.
When one material is rubbed against another, electrons jump readily from one to the other but protons do not. Why is this? (Think in atomic terms.)
Electrons are easily dislodged from the outer regions of atoms, but protons are held tightly within the nucleus.
Why is charge usually transferred by electrons rather than by protons?
Electrons are loosely bond on the outside of atoms, whereas protons are very tightly bound within the atomic nuclei.
What happens to electrons in any charging process?
Electrons transferred from one place to another.
What term is used for "electricity at rest"?
Electrostatics
When combing your hair, you scuff electrons from your hair onto the comb. Is your hair then positively or negatively charged? How about the comb?
Excess electrons rubbed from your hair leave it with a positive charge; excess electrons on the comb give it a negative charge.
Demonstrate charging by friction and discharging from points with a friend who stands at the far end of a carpeted room. Scuff your way across the rug until your noses are close together. This can be a delightfully tingling experience, depending on how dry the air is and how pointed your noses are.
In dry climates this is common nuisance!
What law of physics tells you that if a current-carrying wire produces a force on a magnet, then a magnet must produce a force on a currentcarrying wire?
Newton's third law, which applies to all forces in nature.
A strong magnet attracts a paper clip to itself with a certain force. Does the paper clip exert a force on the strong magnet? If not, why not? If so, does it exert as much force on the magnet as the magnet exerts on it? Defend your answers.
Newtons 3rd Law again: Yes, the paper clip as part of the interaction, certainly does exert a force on the magnet- just as much as the magnet pulls on it. The magnet and the paper clip pull equally on each other.
Are wires needed in Maxwell's view of Faraday's law?
No wires are needed
A balloon may easily be charged to several thousand volts. Does that mean it has several thousand joules of energy? Explain.
No, Several thousand volts if different from the ratio several thousand volts per coulomb. Voltage is measured in volts; voltage/coulombs is energy and measured in joules. Several thousand joules per coulomb isn't much energy if you have a small fraction of a coulomb.
Does the transmission of electric energy require electric conductors between the source and receiver? Cite an example to defend your answer.
No, wires are needed. Personal electronics arrest to this.
Is electromagnetic induction (EMI) an energy source?
No. EMI is not a source of energy but a method of transforming mechanical energy into electric energy. Work must be done to produce energy by EMI.
The voltage has been stepped up, and the current has been stepped down. Ohm's law says that increased voltage will produce increased current. Is there a contradiction here, or doesn't Ohm's law apply to circuits that have transformers?
Ohm's law is alive and well! The current in the secondary equals the induced voltage divided by the load (resistance), but is smaller only when compared with the larger current drawn by the primary. IVprimary = IVsecondary. Interestingly, there is no conventional resistance in the primary, only resistance to the transfer of energy to the secondary (see Figure 25.4).
Name two kinds of practical "electric pumps."
One kind is the battery; another is a generator.
Do both electrical forces and magnetic forces depend on motion?
Only the magnetic force requires motion.
Why will a magnet attract an ordinary nail or paper clip but not a wooden pencil?
Opposing poles in the nails and in the paper clip are into alignment. A wooden pencil does not have a charged domain that would atttract the magnet.
What is the sign of the charge of the particle in this case?
Positive
What is the power in the secondary coil?
Power = voltage × current = 200 V × 4 A = 800 W.
What name is given to the rate at which energy is transferred?
Power is the rate at which energy is transferred.
A video game console requires 6 V to operate correctly. A transformer allows the device to be powered from a 120-V outlet. If the primary has 500 turns, show that the secondary should have 25 turns.
Primary Voltage Secondary Voltage ------------------ = ---------------------- Number of turns Number of turns 25 turns
Calculate the power of a hair dryer that operates on 120 V and draws a current of 10 A.
Refrigerator magnets have Nero strips of alternating north and south poles. The magnets are strong enough to hold sheets of paper against a refrigerator door, but day have a very niche short range because the north and south poles cancel a short distance from the magnetic surface.
`What is different about the magnetic poles of common refrigerator magnets and those of common bar magnets?
Refrigerator magnets have strips of alternating north and south poles. Very short distance because the opposing poles cancel each other at a short distance from the magnetic field.
What are magnetic pole reversals?
Reversal of north and south poles, common throughout Earth's history.
Many dry cereals are fortified with iron, which is added to the cereal in the form of small iron particles. How might these particles be separated from the cereal?
Separation is easy with a magnet
If electrons were positive and protons were negative, discuss whether Coulomb's law would be written the same or differently.
The law would be written the same.
Why does an iron core increase the magnetic induction of a coil of wire?
The magnetic domains that become aligned in the iron core contribute to the overall magnetic field of the coil and therefore increases its magnetic induction.
In Chapter 22, we learned that the direction of the electric field about a point charge is radial to the charge. What is the direction of the magnetic field surrounding a current-carrying wire?
The magnetic field take the form of concentric circles about a current-carrying wire.
A transformer has an input of 6 V and an output of 36 V. If the input is changed to 12 V, show that the output would be 72 V.
The transformer steps up voltage by the factor 36/6 = 6. Therefore the input would be stepped up up to 6 * 12 V = 72 V
Why are materials such as glass and rubber good insulators?
The valence electrons are not lose, they are fixed and which results to them being poor conductors.
A loudspeaker consists of a cone attached to a current-carrying coil located in a magnetic field. What is the relationship between vibrations in the current and vibrations of the cone?
The vibrations in both the coil and the speaker cone have identical frequency at any instant/
Neon signs require about 12,000 V for their operation. Consider a neon-sign transformer that operates off 120-V lines. How many more turns should be on the secondary than on the primary?
The voltage stepped up would be (12,000 V) / (120 V) = 100. So there should be 100 times as many turns on the secondary as compared with the primary
If twice as many coulombs were in the test charge near the charged sphere in Figure 22.22b, how would the electric potential energy of the test charge relative to the sphere be affected? How would its electric potential be affected?
Twice as many coulombs means that the test charge has twice as much electric potential energy (because twice as much work put the charge at that location). But the electric potential would not be affected. Electric potential (measured in volts) is different from electric potential energy (measured in joules). Be sure you understand this before you study further
When 110 volts are impressed across a 22-ohm resistor, the current in the resistor is a.5 A. b.10 A. c.132 A. d.2420 A.
a.5 A.
When a negatively charged balloon is placed against a wooden door, positive charges in the wall are a.attracted to the balloon. b.repelled from the balloon. c.too bound to negative charges in the door to have any effect. d.neutralized.
a.attracted to the balloon.
Voltage will be induced in a wire loop when a magnetic field within that loop a.changes. b.aligns with the electric field. c.is at right angles to the electric field. d.converts to magnetic energy.
a.changes.
The magnetic field lines about a current-carrying wire form a.circles. b.radial lines. c.eddy currents. d.spirals.
a.circles.
When you double the voltage in a simple electric circuit, you double the a.current. b.resistance. c.Both of these. d.None of these.
a.current Explanation: This is straight-forward Ohm's law. Current = voltage/resistance.
The electric current in a copper wire is normally composed of a.electrons. b.protons. c.ions. d.All or any of these.
a.electrons. Explanation: Although current can consist of protons and ions, in a copper wire current consists of electrons appropriately called conduction electrons.
The major difference between a motor and generator is a.input and output. b.direction of windings of coils. c.that one uses ac, and the other dc. d.primarily cosmetic.
a.input and output. Explanation: The main difference between a motor and a generator is the roles of input and output, which is opposite for each.
A step-down transformer has a greater number of coils on the a.input side. b.output side. c.side with lower power. d.None of the above.
a.input side. Comment: The fewer number of coils on the output side steps voltage down.
Electrons on the surface of a conductor will arrange themselves such that the electric field a.inside cancels to zero. b.follows the inverse-square law. c.tends toward a state of minimum energy. d.is shielded from external charges.
a.inside cancels to zero.
An electric meter operates on the principal that a current-carrying wire or coil will a.interact with a magnetic field. b.deflect away from a magnetic field. c.be attracted to a magnet. d.induce a magnetic field when near a magnet.
a.interact with a magnetic field.
The flow of charge in an electric circuit is much a.like the flow of water in a system of pipes. b.different than water flow in pipes. c.like an electric valve. d.like an electric pump.
a.like the flow of water in a system of pipes.
A superconducting electromagnet makes use of a.low-resistance coils of wire. b.super-high temperatures. c.high-speed electrons. d.close packing of high-density magnetic domains.
a.low-resistance coils of wire.
To minimize heat losses for power transported across the countryside, it is best that current in the wires is a.low. b.high. c.not too low and not too high. d.replaced with voltage.
a.low. Comment: High amperage produces large heat losses. So power at low current (and correspondingly high voltage) means less heat loss.
In a circuit with two lamps in parallel, if the current in one lamp is 2 amperes, the current in the battery is a.half, 1 A. b.2 A. c.more than 2 A. d.Not enough information to say.
a.more than 2 A. Explanation: Current in the battery will be the sum of currents in the two branches.
When you scuff electrons off a rug with your shoes, your shoes become a.negatively charged. b.positively charged. c.ionic. d.electrically neutral.
a.negatively charged.
A magnetic force acting on a beam of electrons can change a.only the direction of the beam. b.only the energy of the electrons. c.both the direction and the energy. d.neither the direction nor the energy.
a.only the direction of the beam.
One way to prevent overloading in your home circuit is to a.operate fewer devices at the same time. b.change the wiring from parallel to series for troublesome devices. c.find a way to bypass the fuse or circuit breaker. d.All of these.
a.operate fewer devices at the same time.
When a magnet is moved into a wire coil, an induced current in the coil produces a magnetic field that a.resists motion of the magnet. b.attracts the magnet. c.has negligible effect on the magnet. d.interferes with the electric field.
a.resists motion of the magnet. Comment: This is shown in Figure 25.4.
Both a motor and a generator operate via a.similar concepts. b.quite different concepts. c.idealized transformers. d.independent energy sources.
a.similar concepts.
The change in potential energy of a charged object depends on a.the work done on it. b.its location. c.its mass. d.both mass and location.
a.the work done on it.
A step-up transformer in an electrical circuit can step up a.voltage. b.energy. c.Both of these. d.None of these.
a.voltage. Explanation: Stepping up energy is a big no-no in energy conservation!
Voltage is electric potential energy per charge measured in units of a.volts. b.joules. c.coulombs. d.amperes.
a.volts.
In a simple circuit consisting of a single lamp and a single battery, when current in the lamp is 2 amperes, the current in the battery is a.half, 1 A. b.2 A. c.dependent on internal battery resistance. d.Not enough information to say.
b.2 A. Comment: It is important to know that the current in the devices in series is also the current in the battery. Current will depend on battery resistance, but if there's 2 A in the circuit, there's 2 A in the battery!
When a pair of charged particles are brought twice as close to each other, the force between them becomes a.twice as strong. b.4 times as strong. c.half as strong. d.one-quarter as strong.
b.4 times as strong. Comment: In accord with the inverse-square law.
Which statement is correct? a.Voltage flows in a circuit. b.Charge flows in a circuit. c.A battery is the source of electrons in a circuit. d.All are correct.
b.Charge flows in a circuit. Explanation: Voltage causes the flow of electrons, and doesn't flow. Charges flow. A battery is a source of energy, not electrons.
The underlying physics of an electric motor is that a.electric and magnetic fields repel each other. b.a current-carrying wire experiences force in a magnetic field. c.like magnetic poles repel each other. d.ac voltage is induced by a changing magnetic field.
b.a current-carrying wire experiences force in a magnetic field. Comment: Answer c is indirect, but answer b is more direct.
When a cloud that is negatively charged on its bottom and positively charged on its top moves over the ground below, the ground acquires a.a negative charge. b.a positive charge. c.no charge since the cloud is electrically neutral. d.an electrically grounded state.
b.a positive charge.
Electric charge will flow in an electric circuit when a.electrical resistance is low enough. b.a potential difference exists. c.the circuit is grounded. d.electrical devices in the circuit are not defective.
b.a potential difference exists.
When you thrust a bar magnet to and fro into a coil of wire, you induce a.direct current. b.alternating current. c.neither dc nor ac. d.alternating voltage only, not current.
b.alternating current. Explanation: Indeed alternating voltage is induced, which produces alternating current, so we can say that current as well as voltage is induced in a metal coil. Hence b is correct.
When you buy a water pipe in a hardware store, the water isn't included. When you buy copper wire, electrons a.must be supplied by you, just as water must be supplied for a water pipe. b.are already in the wire. c.may fall out, which is why wires are insulated. d.None of these.
b.are already in the wire.
For magnets, like poles repel each other and unlike poles a.also repel each other. b.attract each other. c.can disappear into nothingness. d.can carry a lot of energy.
b.attract each other.
A negative ion has more a.electrons than neutrons. b.electrons than protons. c.protons than electrons. d.neutrons than protons plus electrons.
b.electrons than protons.
The difference between dc and ac in electrical circuits is that in dc the electrons a.flow steadily in one direction only. b.flow in one direction only. c.steadily flow to and fro. d.flow to and fro.
b.flow in one direction only.
According to Coulomb's law, the force between a pair of charged particles that are brought closer together a.decreases. b.increases. c.increases only if the charges are of the same sign. d.increases only if the charges are of opposite signs.
b.increases.
A bar magnet that holds a chain of paper clips illustrates a.magnetic field displacement. b.magnetic induction. c.electromagnetic induction. d.Faraday's law.
b.magnetic induction.
When we say charge is conserved, we mean that charge can a.be saved, like money in a bank. b.not be created or destroyed. c.be created or destroyed, but only in nuclear reactions. d.take equivalent forms.
b.not be created or destroyed. Explanation: Electric charge cannot be created or destroyed. It can only be transferred from one place to another.
The purpose of a transformer is to transform a.and create energy. b.power at one voltage to the same power at another voltage. c.current from one place to another. d.voltage to useful applications.
b.power at one voltage to the same power at another voltage. Explanation: Answers c or d are not the best choices, as is b, the more direct answer. If you answered a, OUCH! ... no device creates energy!
The fact that electric and magnetic fields regenerate each other is important in a.burglar alarms. b.radio broadcasting. c.metal detectors. d.All of these.
b.radio broadcasting. Explanation: A general answer would be electromagnetic waves, of which only radio broadcasting qualifies.
Lighting a lamp via electromagnetic induction a.bypasses the need of work input. b.requires work input. c.may or may not require work input depending on efficiency. d.produces a low-heat lamp.
b.requires work input. Comment: Never forget an important fundamental of physics: Work is necessary to transform energy, whether the means of doing so is electromagnetic induction or otherwise.
Normally, a rubber balloon charged to thousands of volts has a relatively a.large amount of charge. b.small amount of energy. c.small electric potential. d.small electric field.
b.small amount of energy.
If it takes 10 newtons of force to hold a 0.1-coulomb particle at rest in an electric field, the strength of the field there is a.1 N/C. b.10 N/C. c.100 N/C. d.1000 N/C.
c.100 N/C.
A capacitor can store a.charge. b.energy. c.Both. d.Neither.
c.Both
Magnet A has twice the field strength of Magnet B. When brought close together, the magnet that pulls harder on the other is a.Magnet A. b.Magnet B. c.Both forces are equally strong. d.dependent on their distances apart.
c.Both forces are equally strong. Explanation: This is Newton's third law-the force that A exerts on B is equal and opposite to the force that B exerts on A.
The amount of current in a circuit depends on the a.voltage across the circuit. b.electrical resistance of the circuit. c.Both of these. d.None of these.
c.Both of these.
The fact that a force is exerted on a current-carrying wire in a magnetic field underlies a.motors. b.electric meters. c.Both of these. d.None of these.
c.Both of these.
Surrounding moving electric charges are a.electric fields. b.magnetic fields. c.Both of these. d.None of these.
c.Both of these. Comment: Don't forget that electric fields surround electric charges!
A magnetic force cannot act on an electron when it a.is at rest. b.moves parallel to magnetic field lines. c.Both of these. d.None of these.
c.Both of these. Explanation: A force is exerted on charged particles only when they move at an angle to magnetic field lines. The force is greatest when motion is at right angles to the magnetic field.
Which device is used to convert ac to a fairly steady dc? a.Diode b.Capacitor c.Both of these. d.None of these.
c.Both of these. Explanation: Although a diode will convert ac to dc, the additional use of a capacitor produces steadiness (as Figure 23.11 in the text shows).
The magnetic domains in a magnet produce a weaker magnet when the magnet is a.heated. b.dropped on a hard surface. c.Either of these. d.None of these.
c.Either of these.
A magnetic force acts most strongly on a current-carrying wire when it a.carries a very large current. b.is perpendicular to the magnetic field. c.Either or both of these. d.None of the above.
c.Either or both of these.
What travels at about the speed of light in an electric circuit? a.Electric charges b.Electric current c.Electric field d.All of these.
c.Electric field Explanation: The electric field in a circuit travels at nearly the speed of light, not the electrons nor the current.
A transformer works by way of a.Coulomb's law. b.Ohm's law. c.Faraday's law. d.Newton's second law.
c.Faraday's law.
The essential physics concept in an electric generator is a.Coulomb's law. b.Ohm's law. c.Faraday's law. d.Newton's second law.
c.Faraday's law.
Which of these lamps is the longer lasting in a common circuit? a.Incandescent lamp b.Compact fluorescent lamp c.Light-emitting diode d.All about the same.
c.Light-emitting diode Comment: More about LEDs in Chapter 30.
To receive an electric shock there must be a.current in one direction. b.moisture in the electrical device being used. c.a difference in potential across part or all of the body. d.high voltage and low body resistance.
c.a difference in potential across part or all of the body.
The drift speed of electrons that make up current in a circuit is a.near the speed of light. b.near the speed of sound. c.about a snail's pace. d.imaginary.
c.about a snail's pace.
Within both a conventional ammeter and voltmeter you will find a a.diode. b.capacitor. c.galvanometer. d.tiny motor.
c.galvanometer.
A superconductor has the property of a.changing from a conductor to an insulator. b.becoming conducting when illuminated with light. c.having a low electrical resistance. d.All of the above.
c.having a low electrical resistance. Comment: Don't confuse a superconductor with a semiconductor!
Apply heat to a copper wire and the resistance of the wire a.decreases. b.remains unchanged. c.increases. d.vanishes with enough heat.
c.increases
Faraday's law is the law a.of energy conservation. b.of charge conservation. c.of electromagnetic induction. d.that connects voltage and resistance to magnetic fields.
c.of electromagnetic induction. Comment: Answer a has merit, but answer c is central. Anaswer d is gibberish.
The source of all magnetism is a.tiny bits of iron. b.tiny domains of aligned atoms. c.the motion of electrons. d.small lodestones.
c.the motion of electrons. Comment: This is true for electrons moving in a beam or electrons spinning or orbiting the nuclei of atoms such as iron.
A strong magnet lifts a paper clip. Compared with the force the magnet exerts on the clip, the force the clip exerts on the magnet is a.much less. b.slightly less. c.the same. d.slightly more.
c.the same. Explanation: This is Newton's third law. In the interaction between the paper clip and the magnet, both forces are equal and opposite.
When a bar magnet is broken in half, the magnetic strength of each piece is a.less than half. b.about half. c.the same. d.somewhat increased.
c.the same. Explanation: This is detailed in Figure 24.7 in the textbook.
The net charge of an atom equals a.the number of protons in its nucleus. b.the number of electrons surrounding its nucleus. c.zero if the atom is electrically neutral. d.always zero.
c.zero if the atom is electrically neutral.
A lamp with a current of 10 A connected to 120 volts consumes a power of a.10 W. b.12 W. c.120 W d.1200 W.
d.1200 W.
Magnetic materials are found in the bodies of a.bees. b.birds. c.bacteria. d.All of these.
d.All of these.
Surrounding our Planet Earth are a.radiation belts. b.cosmic rays. c.gravitational and magnetic fields. d.All of these.
d.All of these.
The magnetic field surrounding Earth a.helps shield us from cosmic rays. b.is likely caused by electric currents in its interior. c.changes over time. d.All of these.
d.All of these.
If you change the magnetic field in a closed loop of wire, you induce in the loop a a.current. b.voltage. c.electric field. d.All of these.
d.All of these. Explanation: Recall how Maxwell generalized Faraday's law to include induction of an electric field.
Electric potential and electric potential energy are a.one and the same in most cases. b.two terms for the same concept. c.Both of these. d.None of these.
d.None of these. Explanation: Electric potential is electric potential energy per charge, a ratio of energy per charge in units joules per coulomb (volts). Electric potential energy has units of joules.
Earth's magnetic poles and geographical poles occur a.atop each other. b.about 1000 miles apart. c.in northern Canada. d.beneath ice caps.
d.beneath ice caps.
In the electric field surrounding a group of charged particles, field strength is greater where field lines are a.thickest. b.longest. c.farthest apart. d.closest.
d.closest.
Magnetic domains occur mainly for materials composed of a.carbon. b.copper. c.silver. d.iron.
d.iron.
A changing electric field can induce a changing a.current loop. b.voltage. c.resonance that produces radio. d.magnetic field.
d.magnetic field. Comment: This was Maxwell's generalization of electromagnetic induction.
If you double both the current and the voltage in a circuit, the power a.remains unchanged if resistance remains constant. b.halves. c.doubles. d.quadruples.
d.quadruples. Explanation: Electric power = current ´ voltage. Doubling both current and voltage corresponds to 4 times as much power.
Unlike Newton's law of gravity, Coulomb's law involves a.force at a distance. b.a proportionality constant. c.an inverse-square law. d.repulsive as well as attractive forces.
d.repulsive as well as attractive forces.
The workings of a transformer are consistent mainly with a.Newton's second law. b.Coulomb's law. c.the conservation of momentum. d.the conservation of energy.
d.the conservation of energy.
The thing that we measure in joules per coulomb is a.electric force. b.electric field. c.electric current. d.voltage.
d.voltage.
When a generator is used to light a lamp, the energy of the lit lamp originates in the a.coils of wire in the generator. b.magnet inside the generator. c.lamp itself. d.work done to turn the coils in the generator.
d.work done to turn the coils in the generator.
In what way are magnetic poles very different from electric charges?
Magnetic poles cannot be isolated; electronic charges can.
What is the source of magnetic force?
Moving source
Magnetic force
(1) Between magnets, it is the attraction of unlike magnetic poles for each other and the repulsion between like magnetic poles. (2) Between a magnetic field and a moving charged particle, it is a deflecting force due to the motion of the particle: The deflecting force is perpendicular to the velocity of the particle and perpendicular to the magnetic field lines. This force is greatest when the charged particle moves perpendicular to the field lines and is smallest (zero) when it moves parallel to the field lines.
Transformer relationship: Primary Voltage Secondary Voltage ------------------ = ---------------------- Number of turns Number of turns The primary of a transformer connected to 120 V has 10 turns. The secondary has 100 turns. Show that the output voltage is 1200 V. This is a step-up transformer.
120 V xV ------ = ----- 10 turns 100 turns 1200 V
A transformer for a laptop computer converts a 120-V input to a 24-V output. Show that the primary coil has five times as many turns as the secondary coil.
5/1 Primary Voltage Secondary Voltage ------------------ = ---------------------- Number of turns Number of turns
The wattage marked on a lightbulb is not an inherent property of the bulb but depends on the voltage to which it is connected, usually 110 V or 120 V. How many amperes flow through a 60-W bulb connected in a 120-V circuit?
A magnets one deuce magnetic domains of a nail or paperclip into alignment in a manner similar to the electrical charge polarization in an insulator when a charged object is near by nearby. Either pull a magnet will induce alignment of domains in the paperclip: attraction results because the pole of the aligned domains closest to the magnet's poles
Insulator
A material that does not contain free charged particles and through which charge does not easily flow.
Superconductors
A material that is a perfect conductor with zero resistance to the flow of electric charge.
Semiconductors
A material with properties that fall between those of a conductor and an insulator and whose resistance can be affected by adding impurities.
What exactly does a step-down transformer step down?
A step-down transformer steps down voltage.
Which of these does a transformer change: voltage, current, energy, power?
A transformer changes voltage and current, but not energy and power.
Why does a transformer require ac?
A transformers operation depends on change- hence alternating current, or AC
What is a transistor composed of, and what are some of its functions?
A transistor is composed of thin layers of semiconducting materials. Its functions include controlling the flow of electrons, amplifying signals, and acting as a switch.
The leaves of a charged electroscope collapse in time. At higher altitudes, they collapse more rapidly. Why is this true? (Hint: The existence of cosmic rays was first indicated by this observation.)
Cosmic rays produce ions in air, which offer a conducting path for the discharge any charged objects. Cosmic ray particles streaming down through the atmosphere are attenuated by radioactive decay and buy absorption to radiation and the ionization are stronger at higher altitudes then at lower altitudes. Charged objects more quickly lose their charge at higher altitudes.
How can a magnet attract a piece of iron that is not magnetized?
Domains in the unmagnetized piece of iron are induced into alignment by the magnetic field of the nearby magnet. See the similarity of this to Figure 22.13 in Chapter 22. Like the pieces of paper that jump to the comb, pieces of iron will jump to a strong magnet when it is brought nearby. But, unlike the pieces of paper, they are not then repelled. Can you think of the reason why?
When you double the charge on only one of a pair of particles, what effect does this have on the force between them? Does the effect depend on the sign of the charge?
Doubling one in charge doubles one charge doubles the force. The magnitude of the force does not depend on the side of the charge.
What is meant by drift velocity?
Drift philosophy is the net velocity of the electrons that make up an electric current.
Electronic types neglect the force of gravity on electrons. To see why, compute the force of Earth's gravity on an electron and compare it with the force exerted on the electron by an electric field of magnitude 10,000 V/m (a relatively small field). The mass and charge of an electron are given on the inside back cover.
Fgrav = mg = (9.1 * 10^-31 kg)(9.8 m/s^2) = 8.9 * 10^12 N. F(elec) = qE = (1.6 *10^-19 C)(10,000 V/m) = 1.6 * 10^-15 N, more than 10^14 times greater than the gravitational force!
In what direction relative to a magnetic field does a charged particle move in order to experience maximum deflecting force? Minimum deflecting force?
Force is maximum when motion is perpendicular to the field; minimum when parallel to the field.
What relative direction between a magnetic field and a current-carrying wire results in the greatest force?
Force is maximum when the field is perpendicular to the field.
Suppose that the charges attracting each other in the preceding problem have equal magnitude. Rearrange Coulomb's law and show that the magnitude of each charge is 2.8 × 10-6 C (2.8 microcoulombs).
From Coulomb's law, the force is given by F= kq^2/ d^2, so the square of the charge is q^2= Fd^2 = (20N)(0.06m)^2 /9.1*10^9N * m^2/C^2. Taking the square root of the charge of this gives q = 2.8 microcosmic
Why are the magnetic fields of superconducting magnets often stronger than those of conventional magnets?
Greater electron flow produces greater magnetic field strength.
Give an example of a common electric dipole.
H2O is an example of a charged dipole.
By whom, and in what setting, was the relationship between electricity and magnetism discovered?
Hans Christian Oersted in a high school experiment that demonstrated the relationship between magnetism and electricity.
Does heating a metal wire increase or decrease its electrical resistance?
Heating a metal wire increases molecular motion in therefore its electrical resistance.
Cosmic rays
High-speed particles that travel throughout the universe.
Examine the electric meter in your house. It is probably in the basement or on the outside of the house. You will see that, in addition to the clock-like dials in the meter, there is a circular aluminum disk that spins between the poles of magnets when electric current goes into the house. The more electric current, the faster the disk turns. The speed of the disk is directly proportional to the number of watts used; for example, it spins 5 times as fast for 500 W as for 100 W. You can use the meter to determine how many watts an electrical device uses. First, make sure that all electrical devices in your home are disconnected (it is okay to ignore electric clocks and other 2-W devices, which will hardly be noticeable). The disk will be practically stationary. Then connect a 100-W bulb and note how many seconds it takes for the disk to make five complete revolutions. The black spot painted on the edge of the disk makes this easy. Disconnect the 100-W bulb and plug in a device of unknown wattage. Again, count the seconds for five revolutions. If it takes the same time, it's a 100-W device; if it takes twice the time, it's a 50-W device; half the time, a 200-W device; and so forth. In this way you can estimate the power consumption of devices fairly accurately.
How the charge moves dictates the direction of its magnetic field. (A magnetic field is a vector quantity) Magnetic fields cancel more in some materials than in others.
What happens to the current in other lamps if one of the lamps in a parallel circuit burns out?
If one lamp burns out, the other lamps are unaffected. This is because current in each branch, according to Ohm's law, is equal to voltage/resistance, and since neither voltage nor resistance is affected in the other branches, the current in those branches is unaffected.
How do the direction of the magnetic force and its effects differ between the motor effect and the generator effect, as shown in Figure 25.7?
In both cases the direction of the magnetic force is perpendicular to the magnetic field to the magnetic field and the motion of charges- but with different results. In the motor effect the magnet force pushes the wires upward. In the generator effect, the wire is pushed downward and the magnetic force pushes electrons in a direction along the wire to produce a current.
What is the major similarity between a galvanometer and a simple electric motor? What is the major difference?
In both, coils are positioned in a magnetic field. A force produces rotation when current passes through the coils. The major difference is that the maximum rotation of the coil in a galvanometer is one half-turn, whereas, in a motor, the coil (wrapped on an armature) rotates through many complete turns—accomplished by alternating the current with each half-turn of the armature.
Why are the tires for trucks carrying gasoline and other flammable fluids manufactured to be electrically conducting?
In earlier times, before truck tires were made to be electrically conducting, chains or wires were commonly dragged along the road surface from the bodies of trucks. Their purpose was to discharge any charge that would otherwise build up because of friction with the air and the road. The electrically conducting tires now in use prevent the buildup of static charge that could produce a spark- especially dangerous for trucks carrying flammable cargo.
If a large enough electric field is applied, even an insulator will conduct an electric current, as is evident in lightning discharges through the air. Explain how this happens, taking into account the opposite charges in an atom and how ionization occurs.
In neutral atom in an electric field is electrically distorted. If the field is strong enough, the dissolution results in ionization, which the charges are torn from each other. The ions then provided conducting paths for an electric current.
Why does the negative rod in Figure 22.7 have the same charge both before and after the spheres are charged, but not when charging takes place, as in Figure 22.8?
In the charging process of Figure 22.7, no contact was made between the negative rod and either of the spheres. In Figure 22.8, however, the rod touched the positively charged sphere. A transfer of charge by contact reduced the negative charge on the rod.
Why is the magnetic field strength greater inside a current-carrying loop of wire than about a straight section of wire?
Inside the loop the lines are more concentrated.
Consider members of a marching band standing at rest. You can set them into motion in two ways: (1) Give the last person in line a shove that cascades to the first person in line. (2) Issue the command "Forward, march." Which of these two is analogous to the way electrons move in a circuit when the switch is closed, and which is analogous to the way sound travels?
Issuing the command "Forward, march" is analogous to the way electrons move when they sense the electric field that energizes the circuit when the switch is closed. One marcher lurching against the other is analogous to the way sound travels.
If a proton at a particular distance from a charged particle is repelled with a given force, by how much will the force decrease when the proton is three times farther away from the particle? When it is five times farther away?
It decreases to 1/9 its original value; to 1/25 its original value.
What is a galvanometer called when it has been calibrated to read current? When it has been calibrated to read voltage?
It is called an ammeter; when calibrated for voltage, a voltmeter
Why is it dangerous to touch the terminals of a high-voltage capacitor even after the charging circuit is turned off?
It is dangerous because the capacitor may still be charged.
In a humorous vein, if you rub a balloon on your hair and put your head to the wall, will it stick to the wall like the balloon would?
It would, if you were an airhead—that is, if the mass of your head were about that of the balloon, so that the force produced would be evident.
When iron-hulled naval ships are built, the location of the shipyard and the orientation of the ship in the shipyard are recorded on a brass plaque permanently attached to the ship. Why?
Just at a nail is magnetized by beating on it, an iron ship is beat up in its manufacture, making a permanent magnet. Its initial magnetic field orientation in which is a factor in subsequent magnetic measures, is in effect recorded on the brass plaque.
What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of charge?
Kate must have a difference in temperature. Charge must be a different in electric potential.
Why are lightning rods normally at a higher elevation than the buildings they protect?
Lightning rods are taller in order to be closer to the clouds, closer to lightning.
Beneath the complexities of electrical phenomena, there lies a fundamental rule from which nearly all other effects stem. What is this fundamental rule?
Like charges repel; opposite charges attract.
Which of these is a unit of power and which is a unit of energy: a watt, a kilowatt, a kilowatt-hour?
Magnetic pole reversals of the north and south poles, common throughout Earth's.
If you push a magnet into a coil connected to a resistor, as shown in Figure 25.4, you'll feel a resistance to your push. Why is this resistance greater in a coil that has more loops?
Simply put, more work is required to provide more energy to be dissipated by more current in the resistor. You can also look at it this way: When you push a magnet into a coil, you cause the coil to become a magnet (an electromagnet). The more loops there are in the coil, the stronger the electromagnet that you produce and the stronger it pushes back against the magnet you are moving. (If the coil's electromagnet attracted your magnet instead of repelling it, energy would be created from nothing and the law of energy conservation would be violated. But that doesn't happen.)
What commonly supplies the energy input to a turbine?
Steam
In the phenomenon of superconductivity, what happens to electrical resistance at low temperatures?
The Electrical resistance disappears.
How does the magnitude of the electrical force compare between a pair of charged particles when they are brought to half their original distance of separation? To one-quarter their original distance? To four times their original distance? (What law guides your answers?)
The Inverse- Square law is a play here. At half the distance the only electric force Field is four times as strong ; at one quarter the distance, 16 times stronger; at four times the distance, one 16th as strong.
Coulomb
The SI unit of electric charge. One coulomb (symbol C) is equal to the total charge of 6.25 × 1018 electrons.
When an electric field is applied to a shallow bath of vegetable oil, why do tiny bits of thread floating in the oil align with the field like compasses in a magnetic field?
The bits of thread become polarized and the electric field, with one hand positive and the other negative come and become the electric counterparts of the north and south poles of the magnetic poles. Opposite forces on the end of the fibers (or compass needle) produces torques that orient the fibers along the field direction.
What happens to the brightness of the light from each lamp in a parallel circuit when more lamps are added in parallel?
The brightness of each lamp is unchanged as other lamps are introduced (or removed).
What happens when a magnetically stored bit of information on a computer disk spins under a reading head that contains a small coil?
The changing magnetic field in the coil induces voltage. In this way, information stored magnetically on the disk is converted to electrical signals.
What is meant by conservation of charge?
The charge cannot be created nor destroyed, only transferred.
If you expend 10 J of work to push a 1-C charge against an electric field, what is its change of voltage?
The charge is 10V. (10J per coulomb is 10V)
How does the charge of an electron differ from the charge of a proton?
The charge of an electron is equal in magnitude, but opposite in sign, to the charge of a proton.
Plastic wrap becomes electrically charged when pulled from its box. As a result, it is attracted to objects such as food containers. Does the wrap stick better to plastic containers or to metal containers? Discuss.
The charged wrapped nicely polarizes nonconducting plastic rather than metal, resulting in better sticking on plastic than metal
How do the charges on opposing plates of a capacitor compare?
The charges are of equal magnitude because the charge taken from one plate is given to the other. That's why the net capacitor is always zero.
What happens to the current in other lamps if one lamp in a series circuit burns out?
The circuit is broken and all lamps will go out.
Electromagnetic induction
The creation of voltage when a magnetic field changes with time. If the magnetic field within a closed loop changes in any way, a voltage is induced in the loop: Voltage induced ∼ area of loops × Δ magnetic fieldΔ ------------------------------------ Δtime Voltage induced ∼ area of loops × Δ magnetic fieldΔ time This is a statement of Faraday's law. (If multiple loops are connected together in a coil, the voltage induced is multiplied by the number of loops.) The induction of voltage is actually the result of a more fundamental phenomenon—generally, the induction of an electric field.
Faraday's law
The induced voltage in a coil is proportional to the product of its numbers of loops, the cross-sectional area of each loop, and the rate at which the magnetic field changes within those loops.
A crystal of salt consists of negative and positive ions. How does the net charge of the negative ions compare with that of the positive ions? Explain.
The crystal as a whole has a zero net charge, so any negative charge in one is countered with as much positive charge in another part. So that net charge of the negative electrons has the same magnitude as the net charge of the ions. (this balancing of positive and negative charges within the crystal is almost, but not, precisely perfect because the crystal can gain or lose a few extra electrons.)
What happens to the current in the battery when more lamps are added in parallel?
The current in the battery increases by an amount that feeds the added branch(es). In the overall circuit, added paths means decreased resistance. (There is resistance in a battery also, which we assume is negligible here.)
Is the current that is produced by a common generator ac or dc?
The current is AC because the induced voltage is AC
Potential Difference
The difference in electric potential between two points, measured in volts (synonymous with voltage difference or simply voltage).
How is the direction of an electric field defined?
The direction of an electric field is the direction of force on a positive charge.
Will either pole of a magnet attract a paper clip? Explain what is happening inside the attracted paper clip. (Hint: Consider Figure 22.13.)
The domains of the domains are induced into alignment in a manner similar to the electrical charge polarization in an insulator when when a charged object is brought nearby. Either pole of the magnet will induce alignment of domains in the paper clip attraction results because the poles of the aligned domains closest to the magnet's pole is always the opposite pole.
When charges mutually repel and distribute themselves on the surface of conductors, what becomes of the electric field inside the conductor?
The electric field inside a conductor cancels to zero
Electric Potential
The electric potential energy per unit of charge, measured in volts; often called voltage: Voltage=electric potential energy/charge
How will the directions of travel compare for the electron and the proton in the preceding problem?
The electron and proton will accelerate in opposite directions.
Imagine a proton at rest a certain distance from a negatively charged plate. It is released and collides with the plate. Then imagine the similar case of an electron at rest the same distance away from a similarly charged but positive plate. In which case will the moving particle have the greater speed when the collision occurs? Why?
The electron will have the greater speed of impact. The force on both is the same and the distance is the same, so the work done by the field is the same and the KE of the particles of the same. But fro the same KE, the particle with the smaller mass, the electron, will have the greater speed.
The 50,000 billion billion (5 × 1022) freely moving electrons in a penny repel one another. Why don't they fly out of the penny?
The electrons are attached to the 50,000 billion billion positively charged protons in the atomic new corner of atoms w/in the penny.
Electrically Potential Energy
The energy a charged object possesses by virtue of its location in an electric field.
What is the error in saying that electrons in a common battery-driven circuit travel at about the speed of light?
The error is that no particle can travel at the speed of light.
What is the error in saying that the source of electrons in a circuit is the battery or generator?
The error is that the source is he conducting wires them self not the power source
Electric Current
The flow of electric charge that transports energy from one place to another, measured in amperes, where 1 A is the flow of 6.25 × 10^18 electrons per second, or 1 coulomb per second.
If you place a free electron and a free proton in the same electric field, how will the forces acting on them compare?
The forces on the electrons and the proton will be equal in magnitude but opposite in in direction.
Two pieces of plastic, a full ring and a half ring, have the same radius and charge density. Which electric field at the center has the greater magnitude? Discuss and defend your answer.
The half ring has the greater electric field at Central because the electric field at the center of the whole ring canceled to zero. The electric field at the center of the half ring is due to multitude of electric vectors, with vertical components canceling and horizontal components adding to produce a result field backpacks horizontally to the right.
The proportionality constant k in Coulomb's law is huge in ordinary units, whereas the proportionality constant G in Newton's law of gravitation is tiny. What does this indicate about the relative strengths of these two forces?
The huge value of the constant k case for electrical force indicates a relatively high huge force between charges, compared with the small gravitational force between masses and the small value of the gravitational constant G.
By what mechanism do the iron filings shown about the magnet in Figure 24.2 align with the magnetic field?
The mechanism of alignment requires two factors: First, each magnet is turned into a tiny magnet by the magnetic field in the bar magnet, each induce domain alignment on the filings. Second, a pair of opposite and equal torques act on each filing when whenever it is not parallel to the magnetic field lines. The torques rotate the filings into alignment with the filed lines like little compass needles.
What produces a magnetic field?
The motion of electric charges produces a magnetic field
What is most commonly the net charge of an atom?
The normal net charge is zero.
Which part of an atom is positively charged, and which part is negatively charged?
The nucleus is positively charged and the electrons surrounding the nucleus are negatively charged.
Two charged particles are projected into a magnetic field that is perpendicular to their velocities. If the particles are deflected in opposite directions, what does this tell you about them?
The particles enter the field moving in the same direction and are deflected inn opposite directions, the charges must be opposites.
Strictly speaking, will a penny be slightly more massive if it has a negative charge or a positive charge? Discuss.
The penny Will be slightly more massive with a negative charge because it will have more electrons then when neutral . If the penny where positively charged, it would be slightly lighter because of the missing electrons.
When a car is moved into a painting chamber, a mist of paint is sprayed around its body. When the body is given a sudden electric charge and mist is attracted to it—presto—the car is quickly and uniformly painted. What does the phenomenon of polarization have to do with this?
The pink particles in the mist all polarized and are therefore
Why isn't it advisable to make a horseshoe magnet from a flexible material?
The poles of the magnet cause the magnet to bend even enough for the poles to touch if the material is flexible enough
Calculate the current in the 240-Ω filament of a bulb connected to a 120-V line. Power = I × V
The pores of the magnets attract each other in a cozy magnet to Bend, even enough the pose to the material is flexible enough.
A negatively charged rod is brought close to some small pieces of neutral paper. The positive sides of the molecules in the paper are attracted to the rod and the negative sides of the molecules are repelled. Why don't these attractive and repulsive forces cancel out?
The positive sides are simply closer to the rod. They therefore experience a greater electrical force than the farther-away negative sides. Hence we say that closeness wins. Can you see that a positive rod would still produce attraction?
Electric Power
The rate of energy transfer, or the rate of doing work; the amount of energy per unit time, which can be computed as the product of current and voltage: Power = current × voltage Electric power is measured in watts (or kilowatts), where 1 W = 1 A × 1 V = 1 J/s.
Charging by Induction
The redistribution of electric charges in and on objects caused by the electrical influence of a charged object close by but not in contact.
Magnetic field
The region of magnetic influence around a magnetic pole or a moving charged particle.
Coulomb's Law
The relationship among electrical force, charge, and distance: Q1Q2 F = k -------- d^2 If the charges are alike in sign, the force is repulsive; if the charges are unlike, the force is attractive.
The proton that is the nucleus of the hydrogen atom attracts the electron that orbits it. Relative to this force, does the electron attract the proton with less force, with more force, or with the same amount of force?
The same amount of force, in accord with Newton's third law—basic mechanics! Recall that a force is an interaction between two things—in this case, between the proton and the electron. They pull on each other equally.
Ohm's Law
The statement that the current in a circuit varies in direct proportion to the potential difference or voltage across the circuit and inversely with the circuit's resistance: Current=voltage/resistance A potential difference of 1 V across a resistance of 1 Ω produces a current of 1 A.
Briskly rub a comb through your hair or on a woolen garment and bring it near a small but smooth stream of running water. Is the stream of water deflected?
The stream is deflected, due to the polarity of water molecules.
Electrostatic
The study of electric charge at rest (not in motion, as in electric currents).
Electrically Polarized
The term applied to an atom or molecule in which the charges are aligned so that one side has a slight excess of positive charge and the other side a slight excess of negative charge.
Charging by contact
The transfer of electric charges between objects by rubbing or simple touching.
In terms of net charge, how does an electrically polarized object differ from an electrically charged object?
Unlike a charged object, a polarized object may have no charge.
What is the voltage at the location of a 0.0001-C charge that has an electric potential energy of 0.5 J (both measured relative to the same reference point)?
Voltage = (0.5J)/0.0001C = 5,000V.
How does wetness affect the resistance of your body?
Wetness lowers your body's electrical resistance.
If the voltage impressed across a circuit is held constant while the resistance doubles, what change occurs in the current?
When the resistance doubles, the current is halved.
From where do the electrons originate that produce an electric shock when you touch a charged conductor?
When you are shocked, your own body is the source of the electrons, not the source of energy imparted to them.
Why is a generator armature harder to rotate when it is connected to a circuit and supplying electric current?
Work must be done to move a current-carrying conductor in a magnetic field. This is true whether the current is externally produced or produced as a result of induction that accompanies the motion of the wire in the field. It's also a matter of energy conservation. There has to be more energy input to have more energy output.
Is it correct to say that an electric motor extends the physics that underlies a galvanometer?
Yes, an electric motor is a more specified galvanometer.
Would the charges induced on spheres A and B of Figure 22.7 necessarily be exactly equal and opposite?
Yes, because each positive charge on sphere A results from an electron taken from A and moved to B. This is like removing bricks from the surface of a brick road and placing them all on the sidewalk. The number of bricks on the sidewalk exactly matches the number of holes in the road. Likewise, the number of extra electrons on B will exactly match the number of "holes" (positive charges) left in A. The positive charge is the result of an absent electron.
Does every magnet necessarily have a north and a south pole?
Yes, just as every coin has two sides, a "head" and a "tail." Some "trick" magnets have more than one pair of poles, but, nevertheless, poles always occur in pairs.
When the magnetic field changes in a coil of wire, voltage in each loop of the coil is induced. Will voltage be induced in a loop if the source of the magnetic field is the coil itself?
Yes. It is called self induction.
Okay, so a potential difference across the ends of a wire produces current. Instead of saying potential difference, can we as well say voltage?
Yes. Recall from Chapter 22 that potential difference and voltage are interchangeable terms—the difference in electric potential between two points in a conducting path. Both are measured in units of volts.
We know that a compass points northward because Earth is a giant magnet. Will the northward-pointing needle point northward when the compass is brought to the Southern Hemisphere?
Yes. The compass aligns with the Earth's magnetic field.
If you scuff electrons onto your feet while you walk across a rug, are you negatively or positively charged?
You have more electrons after you scuff your feet, so you are negatively charged (and the rug is positively charged).
What is the net charge of a charged capacitor?
Zero, because the charges on its two plates are equal in number and opposite in sign. Even when the capacitor is discharged—say, by providing a path for charge flow between the oppositely charged plates—the net charge of the capacitor remains zero because then each plate has zero charge.
What is the magnitude of the electric field inside the dome of a charged Van de Graaff generator?
Zero, whether or not charge is on the outside.
Electrical Resistence
he property of a material that resists electric current, measured in ohms (Ω).
All atoms have moving electric charges. Why, then, aren't all materials magnetic?
how the charge moves dictates the direction of its magnetic field of its magnetic field.
